A format string consists of zero or more “format units.” A format unit
describes one Python object; it is usually a single character or a
parenthesized sequence of format units. With a few exceptions, a format unit
that is not a parenthesized sequence normally corresponds to a single address
argument to these functions. In the following description, the quoted form is
the format unit; the entry in (round) parentheses is the Python object type
that matches the format unit; and the entry in [square] brackets is the type
of the C variable(s) whose address should be passed.

These formats allow accessing an object as a contiguous chunk of memory.
You don’t have to provide raw storage for the returned unicode or bytes
area. Also, you won’t have to release any memory yourself, except with the
es, es#, et and et# formats.

s (string or Unicode) [const char *]

Convert a Python string or Unicode object to a C pointer to a character
string. You must not provide storage for the string itself; a pointer to
an existing string is stored into the character pointer variable whose
address you pass. The C string is NUL-terminated. The Python string must
not contain embedded NUL bytes; if it does, a TypeError exception is
raised. Unicode objects are converted to C strings using the default
encoding. If this conversion fails, a UnicodeError is raised.

This variant on s stores into two C variables, the first one a pointer
to a character string, the second one its length. In this case the Python
string may contain embedded null bytes. Unicode objects pass back a
pointer to the default encoded string version of the object if such a
conversion is possible. All other read-buffer compatible objects pass back
a reference to the raw internal data representation.

Starting with Python 2.5 the type of the length argument can be controlled
by defining the macro PY_SSIZE_T_CLEAN before including
Python.h. If the macro is defined, length is a Py_ssize_t
rather than an int.

s* (string, Unicode, or any buffer compatible object) [Py_buffer]

Similar to s#, this code fills a Py_buffer structure provided by the
caller. The buffer gets locked, so that the caller can subsequently use
the buffer even inside a Py_BEGIN_ALLOW_THREADS block; the caller is
responsible for calling PyBuffer_Release with the structure after it
has processed the data.

New in version 2.6.

z (string, Unicode or None) [const char *]

Like s, but the Python object may also be None, in which case the C
pointer is set to NULL.

Convert a Python Unicode object to a C pointer to a NUL-terminated buffer
of 16-bit Unicode (UTF-16) data. As with s, there is no need to
provide storage for the Unicode data buffer; a pointer to the existing
Unicode data is stored into the Py_UNICODE pointer variable whose
address you pass.

u# (Unicode) [Py_UNICODE *, int]

This variant on u stores into two C variables, the first one a pointer
to a Unicode data buffer, the second one its length. Non-Unicode objects
are handled by interpreting their read-buffer pointer as pointer to a
Py_UNICODE array.

This variant on s is used for encoding Unicode and objects convertible
to Unicode into a character buffer. It only works for encoded data without
embedded NUL bytes.

This format requires two arguments. The first is only used as input, and
must be a constchar* which points to the name of an encoding as
a NUL-terminated string, or NULL, in which case the default encoding is
used. An exception is raised if the named encoding is not known to Python.
The second argument must be a char**; the value of the pointer
it references will be set to a buffer with the contents of the argument
text. The text will be encoded in the encoding specified by the first
argument.

PyArg_ParseTuple() will allocate a buffer of the needed size, copy
the encoded data into this buffer and adjust *buffer to reference the
newly allocated storage. The caller is responsible for calling
PyMem_Free() to free the allocated buffer after use.

This variant on s# is used for encoding Unicode and objects convertible
to Unicode into a character buffer. Unlike the es format, this variant
allows input data which contains NUL characters.

It requires three arguments. The first is only used as input, and must be
a constchar* which points to the name of an encoding as a
NUL-terminated string, or NULL, in which case the default encoding is
used. An exception is raised if the named encoding is not known to Python.
The second argument must be a char**; the value of the pointer
it references will be set to a buffer with the contents of the argument
text. The text will be encoded in the encoding specified by the first
argument. The third argument must be a pointer to an integer; the
referenced integer will be set to the number of bytes in the output buffer.

There are two modes of operation:

If *buffer points a NULL pointer, the function will allocate a buffer
of the needed size, copy the encoded data into this buffer and set
*buffer to reference the newly allocated storage. The caller is
responsible for calling PyMem_Free() to free the allocated buffer
after usage.

If *buffer points to a non-NULL pointer (an already allocated buffer),
PyArg_ParseTuple() will use this location as the buffer and
interpret the initial value of *buffer_length as the buffer size. It
will then copy the encoded data into the buffer and NUL-terminate it. If
the buffer is not large enough, a TypeError will be set.
Note: starting from Python 3.6 a ValueError will be set.

In both cases, *buffer_length is set to the length of the encoded data
without the trailing NUL byte.

Store a Python object (without any conversion) in a C object pointer. The
C program thus receives the actual object that was passed. The object’s
reference count is not increased. The pointer stored is not NULL.

O! (object) [typeobject, PyObject *]

Store a Python object in a C object pointer. This is similar to O, but
takes two C arguments: the first is the address of a Python type object,
the second is the address of the C variable (of type PyObject*)
into which the object pointer is stored. If the Python object does not
have the required type, TypeError is raised.

O& (object) [converter, anything]

Convert a Python object to a C variable through a converter function.
This takes two arguments: the first is a function, the second is the
address of a C variable (of arbitrary type), converted to void*.
The converter function in turn is called as follows:

status=converter(object,address);

where object is the Python object to be converted and address is the
void* argument that was passed to the PyArg_Parse*()
function. The returned status should be 1 for a successful
conversion and 0 if the conversion has failed. When the conversion
fails, the converter function should raise an exception and leave the
content of address unmodified.

S (string) [PyStringObject *]

Like O but requires that the Python object is a string object. Raises
TypeError if the object is not a string object. The C variable may
also be declared as PyObject*.

U (Unicode string) [PyUnicodeObject *]

Like O but requires that the Python object is a Unicode object. Raises
TypeError if the object is not a Unicode object. The C variable may
also be declared as PyObject*.

t# (read-only character buffer) [char *, int]

Like s#, but accepts any object which implements the read-only buffer
interface. The char* variable is set to point to the first byte
of the buffer, and the int is set to the length of the buffer.
Only single-segment buffer objects are accepted; TypeError is raised
for all others.

w (read-write character buffer) [char *]

Similar to s, but accepts any object which implements the read-write
buffer interface. The caller must determine the length of the buffer by
other means, or use w# instead. Only single-segment buffer objects are
accepted; TypeError is raised for all others.

w# (read-write character buffer) [char *, Py_ssize_t]

Like s#, but accepts any object which implements the read-write buffer
interface. The char* variable is set to point to the first byte
of the buffer, and the Py_ssize_t is set to the length of the
buffer. Only single-segment buffer objects are accepted; TypeError
is raised for all others.

w* (read-write byte-oriented buffer) [Py_buffer]

This is to w what s* is to s.

New in version 2.6.

(items) (tuple) [matching-items]

The object must be a Python sequence whose length is the number of format
units in items. The C arguments must correspond to the individual format
units in items. Format units for sequences may be nested.

Note

Prior to Python version 1.5.2, this format specifier only accepted a
tuple containing the individual parameters, not an arbitrary sequence.
Code which previously caused TypeError to be raised here may now
proceed without an exception. This is not expected to be a problem for
existing code.

It is possible to pass Python long integers where integers are requested;
however no proper range checking is done — the most significant bits are
silently truncated when the receiving field is too small to receive the value
(actually, the semantics are inherited from downcasts in C — your mileage
may vary).

A few other characters have a meaning in a format string. These may not occur
inside nested parentheses. They are:

|

Indicates that the remaining arguments in the Python argument list are
optional. The C variables corresponding to optional arguments should be
initialized to their default value — when an optional argument is not
specified, PyArg_ParseTuple() does not touch the contents of the
corresponding C variable(s).

:

The list of format units ends here; the string after the colon is used as
the function name in error messages (the “associated value” of the
exception that PyArg_ParseTuple() raises).

;

The list of format units ends here; the string after the semicolon is used
as the error message instead of the default error message. : and
; mutually exclude each other.

Note that any Python object references which are provided to the caller are
borrowed references; do not decrement their reference count!

Additional arguments passed to these functions must be addresses of variables
whose type is determined by the format string; these are used to store values
from the input tuple. There are a few cases, as described in the list of
format units above, where these parameters are used as input values; they
should match what is specified for the corresponding format unit in that case.

For the conversion to succeed, the arg object must match the format and the
format must be exhausted. On success, the PyArg_Parse*() functions
return true, otherwise they return false and raise an appropriate exception.
When the PyArg_Parse*() functions fail due to conversion failure in
one of the format units, the variables at the addresses corresponding to that
and the following format units are left untouched.

Function used to deconstruct the argument lists of “old-style” functions
— these are functions which use the METH_OLDARGS parameter
parsing method. This is not recommended for use in parameter parsing in
new code, and most code in the standard interpreter has been modified to no
longer use this for that purpose. It does remain a convenient way to
decompose other tuples, however, and may continue to be used for that
purpose.

A simpler form of parameter retrieval which does not use a format string to
specify the types of the arguments. Functions which use this method to
retrieve their parameters should be declared as METH_VARARGS in
function or method tables. The tuple containing the actual parameters
should be passed as args; it must actually be a tuple. The length of the
tuple must be at least min and no more than max; min and max may be
equal. Additional arguments must be passed to the function, each of which
should be a pointer to a PyObject* variable; these will be filled
in with the values from args; they will contain borrowed references. The
variables which correspond to optional parameters not given by args will
not be filled in; these should be initialized by the caller. This function
returns true on success and false if args is not a tuple or contains the
wrong number of elements; an exception will be set if there was a failure.

This is an example of the use of this function, taken from the sources for
the _weakref helper module for weak references:

Create a new value based on a format string similar to those accepted by
the PyArg_Parse*() family of functions and a sequence of values.
Returns the value or NULL in the case of an error; an exception will be
raised if NULL is returned.

Py_BuildValue() does not always build a tuple. It builds a tuple
only if its format string contains two or more format units. If the format
string is empty, it returns None; if it contains exactly one format
unit, it returns whatever object is described by that format unit. To
force it to return a tuple of size 0 or one, parenthesize the format
string.

When memory buffers are passed as parameters to supply data to build
objects, as for the s and s# formats, the required data is copied.
Buffers provided by the caller are never referenced by the objects created
by Py_BuildValue(). In other words, if your code invokes
malloc() and passes the allocated memory to Py_BuildValue(),
your code is responsible for calling free() for that memory once
Py_BuildValue() returns.

In the following description, the quoted form is the format unit; the entry
in (round) parentheses is the Python object type that the format unit will
return; and the entry in [square] brackets is the type of the C value(s) to
be passed.

The characters space, tab, colon and comma are ignored in format strings
(but not within format units such as s#). This can be used to make
long format strings a tad more readable.

s (string) [char *]

Convert a null-terminated C string to a Python object. If the C string
pointer is NULL, None is used.

s# (string) [char *, int]

Convert a C string and its length to a Python object. If the C string
pointer is NULL, the length is ignored and None is returned.

z (string or None) [char *]

Same as s.

z# (string or None) [char *, int]

Same as s#.

u (Unicode string) [Py_UNICODE *]

Convert a null-terminated buffer of Unicode (UCS-2 or UCS-4) data to a
Python Unicode object. If the Unicode buffer pointer is NULL,
None is returned.

u# (Unicode string) [Py_UNICODE *, int]

Convert a Unicode (UCS-2 or UCS-4) data buffer and its length to a
Python Unicode object. If the Unicode buffer pointer is NULL, the
length is ignored and None is returned.

i (integer) [int]

Convert a plain C int to a Python integer object.

b (integer) [char]

Convert a plain C char to a Python integer object.

h (integer) [short int]

Convert a plain C shortint to a Python integer object.

l (integer) [long int]

Convert a C longint to a Python integer object.

B (integer) [unsigned char]

Convert a C unsignedchar to a Python integer object.

H (integer) [unsigned short int]

Convert a C unsignedshortint to a Python integer object.

I (integer/long) [unsigned int]

Convert a C unsignedint to a Python integer object or a Python
long integer object, if it is larger than sys.maxint.

k (integer/long) [unsigned long]

Convert a C unsignedlong to a Python integer object or a
Python long integer object, if it is larger than sys.maxint.

L (long) [PY_LONG_LONG]

Convert a C longlong to a Python long integer object. Only
available on platforms that support longlong.

K (long) [unsigned PY_LONG_LONG]

Convert a C unsignedlonglong to a Python long integer object.
Only available on platforms that support unsignedlonglong.

n (int) [Py_ssize_t]

Convert a C Py_ssize_t to a Python integer or long integer.

New in version 2.5.

c (string of length 1) [char]

Convert a C int representing a character to a Python string of
length 1.

Pass a Python object untouched (except for its reference count, which is
incremented by one). If the object passed in is a NULL pointer, it is
assumed that this was caused because the call producing the argument
found an error and set an exception. Therefore, Py_BuildValue()
will return NULL but won’t raise an exception. If no exception has
been raised yet, SystemError is set.

S (object) [PyObject *]

Same as O.

N (object) [PyObject *]

Same as O, except it doesn’t increment the reference count on the
object. Useful when the object is created by a call to an object
constructor in the argument list.

O& (object) [converter, anything]

Convert anything to a Python object through a converter function.
The function is called with anything (which should be compatible with
void*) as its argument and should return a “new” Python
object, or NULL if an error occurred.

(items) (tuple) [matching-items]

Convert a sequence of C values to a Python tuple with the same number of
items.

[items] (list) [matching-items]

Convert a sequence of C values to a Python list with the same number of
items.

{items} (dictionary) [matching-items]

Convert a sequence of C values to a Python dictionary. Each pair of
consecutive C values adds one item to the dictionary, serving as key and
value, respectively.

If there is an error in the format string, the SystemError exception
is set and NULL returned.